services: virtual-build-machine: Use a non-volatile root file system.

This is a followup to e0ade40c2b, which
didn’t have the desired effect.

* gnu/services/virtualization.scm (virtual-build-machine-default-image):
Add ‘volatile-root?’ field.

Change-Id: I1a622f22cdd0f8f83860a6eb4a852518db3b46b7

gnu/services/virtualization.scm

@@ -1239,6 +1239,7 @@ authpriv.*;auth.info                    /var/log/secure
                                 (virtual-build-machine-name config)))
            (format 'compressed-qcow2)
            (partition-table-type 'mbr)
+           (volatile-root? #f)
            (shared-store? #f)
            (size %default-virtual-build-machine-image-size)
            (partitions (match (image-partitions base)